I am wondering if there are any links to articles or resources available online, or explanations that you can give, that would help me to understand the concept of a public key ring, and why I might (or might not) want to publish my public key to one or another of them.

In other words, I have a basic understanding (I hope) of how PKI works and why I'd publish my public key to a keyserver so people can find me, but I'm unclear on what these different rings mean, or what their application is.

I'm wondering what would be a good analogy for them? Like, Google+ circles? I am looking for a better understanding of this concept.

Hello, welcome to crypto.se :) Just a friendly note from a mod - your original title was "what are some advantages...". Normally, we see titles like that and expect a poor question (they're usually asking things that are off topic) but you had a good one underneath! My advice: don't be afraid to make your question titles specific to what you're asking - that way people are much more likely to read it and answer :) Whilst I'm here, there's an edit link under the question if you want to improve on my title - feel free to do so.
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user46Feb 23 '12 at 21:32

Thanks :) I like these two answers too, but don't have the reputation to upvote them yet. Thanks guys.
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Rob TurkSep 18 '12 at 18:49

Yes. It's similar to your G+ Circles or Facebook lists. Keyring is a directory of known encryption keys or Public Keys. If you are sending a message to someone that doesn't have access to this key ring, you can't send them an encrypted message.